1. Field of the Invention
The present invention relates to an image forming apparatus and, more specifically, to a multi-color image forming apparatus for forming a multi-color image by forming toner images in different colors sequentially on an image retainer so that it can be used in the field of electrophotography.
2. Description of the Prior Art
In the prior art, in order to form a multi-color image by the electrophotography, for example, a series of steps of charge, exposure (image writing), development and transfer are repeated for each color component to transfer toner images in individual colors onto a recording paper in a superposed manner. For example, electrostatic latent images are formed separately at the respective steps by separated colors which are obtained through color separation filters of such as blue, green and red and are developed in Yellow, Magenta and cyan or, if necessary, black toner to form monochromatic toner images. These toner images are transferred in the formed order to the recording paper to form the multi-color image. In this multi-color forming method, however, there arise difficulties:
(1) a transfer to recording paper becomes necessary at the end of development of each color to enlarge the size of the apparatus and to elongate the time period for the image formation; and
(2) failure of registration due to the repeated operations becomes liable to occur.
Therefore, there has been proposed a multi-color image forming method for eliminating those difficulties by separating the colors of optical information of an original into time-series signals by means of a CCD solid-state image sensor through color filters, by developing a plurality of toner images on a common photosensitive member in a superposed manner to reduce the transfer steps to one. However, this method is also encountered by a trouble that a toner image developed at a previous step is disturbed at a subsequent developing step or that toner in a developer at a preceding step is mixed with a developer at a succeeding step to break the color balance of the multi-color image.
In order to avoid this trouble, there has also been developed a method for forming a multi-color image by adopting a method in which a bias having a superposed a.c. component is applied to a developing device at a second or later developments to fly toner onto an electrostatic latent image formed on an image retainer. According to this method, the developer layer will not rub the toner image or images formed at the preceding step so that no image disturbance will occur.
The principle of this multi-color image forming method will be described in the following with reference to a flow chart of FIG. 15. FIG. 15 shows changes in the surface potential of the photosensitive member and takes up a case in which the charge polarity is positive. Reference letters PH indicate an exposed portion of the photosensitive member; letters DA, an unexposed portion of the photosensitive member; and letters DUP, the rise of potential, which is caused as a result that positively charged toner T sticks to the exposed portion PH in a first development.
The photosensitive member is charged evenly by a scorotron charger to have a constant positive surface potential E, as shown in (a). Next, a first image exposure is effected by means of an exposing source such as a laser, a cathode ray tube or an LED so that the potential of the exposed portion PH drops, as shown in (b), in accordance with the quantity of light. An electrostatic latent image thus formed is developed by means of a developing device to which is applied a positive bias substantially equal to the surface potential E of the unexposed portion. As a result, as shown in (c), the positively charged toner T sticks to the exposure portion having a lower potential to form the first toner image T. The region formed with that toner image has the potential rise DUP as a result of the stick of the positively charged toner T, but will not have the same potential as that of the unexposed portion DA. Next, the photosensitive member surface formed with the first toner image is subjected to a second charge by a charger so that it takes the uniform surface potential E despite whether the toner T is present or absent. This is shown in (d). The surface of that photosensitive member is subjected to a second image exposure to form an electrostatic latent image (as shown in (e)), and a positively charged toner image T' in a color different from that of the toner T is developed like the step (c) to form a second toner image. This is shown in (f). The process thus far described is repeated to form a multi-color toner image on the photosensitive member. It is transferred onto the recording paper and is further heated and processed for fixing to obtain a multi-color recorded image. In this case, the photosensitive member is cleaned through cleaning of the toner and charges having remained on its surface and is used for forming a next multi-color image. On the other hand, there is another method by which a toner image is fixed on a photosensitive member in a different manner.
In the method described with reference to FIG. 15, it is desirable that at least the developing step of (f) be conducted such that the developer layer is out of contact with the photosensitive member surface.
Incidentally, in the aforementioned multi-color image forming method, the second and later charges can be omitted. In case where charging is repeated each time of the image formation, a charge eliminating step may be incorporated before the charging step. On the other hand, the exposure source for the image exposure may be either identical or different at each time.
In the electrophotography, a gas or semiconductor laser beam, an LED, a CRT or a liquid crystal is used as the image exposing means.
As the latent image forming method for forming the multi-color image, there can be used in addition to the aforementioned electrophotographic method a method, in which charges are implanted directly into an image forming member by means of a multi-stylus electrode to form an electrostatic latent image, or a method in which a magnetic latent image is formed by means of a megnetic head.
In the non-contact developing described above, it is preferable to apply an a.c. bias to the developing device so as to supply the latent image with sufficient toner. Moreover, it is desirable that the a.c. bias be applied exclusively to the developing device containing the toner in a color to be developed. If the a.c. bias is applied to the developing device having no contribution to the development, the toner in that developing device will stick to the latent image surface to make the color turbid, or the toner in a different color having stuck to the image retainer will mix with the developing device to adversely affect the color reproduction.
This makes it necessary to turn on and off the a.c. bias each time of development for each developing device. At this piont time, however, switching noises may be generated to malfunction the latent image forming means to invite missings of dots or breakage of the image data.
In this multi-color image forming apparatus, on the other hand, it is desired to provide drive means for turning developing sleeves or inside magnetic rolls separately for the individual developing devices. This is explained by the following reasons in case a common drive source is to be used:
(1) Each developing device has to be equipped with drive force transmitting means such as belt or chain; and
(2) Means for connecting and disconnecting the drive force individually (such as clutch) is required. To meet these requirements, the apparatus becomes complicated, and the drive source is required to endure a high torque. In case the drive sources are provided individually, on the contrary, the drive force transmission means can be dispensed with, and each drive source may be small enough for actuating one developing device. In this case, too, it is desired that each developing device be driven only for a time period of contributing to the development. If the developing device out of the development is driven, its toner will stick to the latent image surface to make the color turbid, or the toner already having stuck to the image retainer will mix.
Therefore, the drive source for each driving device has to be turned on and off for each development. In this case, however, there is a fear that the switching noises will be generated to result in malfunctions of the latent image forming means, as has been described hereinbefore. Causes of the switching noises are not clearly found out but seem to come from the electromagnetic waves, the changes in the earth potential or the stray capacity, which are generated when the drive sources are turned on and off.
In addition, the sources for malfunctions of the latent image forming means are located in portions for applying high voltage such as charge electrodes or transfer electrodes, for example, in the case of the electrophotography.
The malfunctions of the latent image forming means due to the aforementioned switching noises are especially liable to occur in the so-called "digital type image formations" in which the image is formed by reading and storing an original image in an image memory and on the basis of the storage of the image memory.
In another multi-color image forming apparatus of the prior art, moreover, the toner images of individual colors formed on the image retainer are transferred sequentially to the recording paper, on which the toner images are superposed. However, this multi-color image forming apparatus is encountered by problems that the necessity for a transfer drum enlarges the size of the apparatus and that the transfer misregistrations of the toner images in the individual colors are caused to make it impossible to form a clear multi-color image.
In the image forming apparatus for forming a plurality of toner images in the superposed manner on the image retainer, however, it is necessary to release the abutment of a blade against the image retainer in the toner image superposing procedures so as to prevent the preceding toner image from being damaged. Here, the cleaning of the residual toner by means of the blade is usually conducted by bringing an elastic plate member into abutment against the surface of the image retainer to scrape off the toner in accordance with the turns of the image retainer. As a result, if the blade abutting against the image retainer is abruptly separated when it is to be released from its abutting state, the toner being scraped is left as it is on the image retainer and is carried out of the cleaning device to raise a problem that it blots the peripheral devices of the image retainer or obstructs formation of a next image.
When the cleaning blade is continuously held in sliding contact with the surface of the image retainer, more specifically, there arises a defect that the retainer surface is damaged while the blade has its edge portion worn so that the cleaning operation cannot be conducted for a long time period. In order to eliminate this defect, there has been proposed a cleaning device in which a blade member is made so movable that it comes into sliding contact with the image retainer only when the image retainer bearing the residual toner left after the transfer comes to the cleaning portion, whereas it is retracted from the image retainer surface for the other time period. This technical means has an effect that it does not damage the image retainer surface but to elongate the lifetime of the blade. In the cleaning device having the blade member to be brought into and out of abutment against the image retainer, however, the scraped toner stored on the edge portion of the blade member when this member is removed from the image retainer is left on the image retainer to raise another problem that it overlaps a next new image to disturb it.